On my website www.pooq.org I had also problems with spam in the forum.

It helped a big deal to rename the POST-variables (which can easiliy be found - they are named something like 'username' and 'password' & '$username' and '$password') by some fantasynames like '$grrrrpfzz' or anythink you like.
(OK, not that easily because of the many modules in the code - but it should be done within an hour)

Additionally not all captchas are save as far as I know - so it is possibly better to create a simple captcha of you own than using a professional but widely spread one.

Of course that will be no help against human spammers (but the topic : "PHPBB Spam Forum - Spammers nowadays choose the first forum, let them spam this one instead of Mango" seems to be no great help either against humans..) but will keep out a lot of spambots.